When I create equine paintings I start with natural horse coat colors and then exaggerate from there but if you want something truly unique then you can select an arbitrary color (purple, teal, etc.) for your equine subject.
For background color I can either contrast the horse with a complimentary color (purple/yellow, red/green, blue/orange) or can stay within the same color family (red, orange, yellow). It is important for me to know your color likes/dislikes or if you are trying to match the painting to decor in your home.
Sorrel - reds
Bay - browns
Strawberry Roan - pinks
Blue Roan - blues
Palomino - yellows w/ lighter yellow mane/tail
Dun - yellows w/ dark yellow mane/tail
Black - dark greys or any dark color (blues, purples)
White - light greys or any light color (pink, blue, yellow)
Images on this page have been used with permission from Ana Marta Loura Equestrian Photography located in Portugal.